The sea serpent was used as a fake monster by the Sea Serpent Smugglers.
It was a large serpentine creature with green scales, sharp teeth, and spikes.
Scooby-Doo! And The Legend Of The Vampire
While Mystery Inc. were pursued by the kelp monster, shark creature and the peg leg captain, Shaggy Rogers accidentally slipped on a pail of water as he and Scooby-Doo landed in the pool. Then, a large sea serpent emerged and roared at them. Scooby accidentally popped it, thus revealing it to be a fake monster. When the three monsters were about to harm Fred Jones and the girls, the fake serpent landed right on the thieves.
- DTV5. Scooby-Doo! And The Legend Of The Vampire (no lines)